With the power OFF....
Pull out the cable (gently wiggle - the cable- if needed) and replace it back in the socket.
This can clean the pins enough to reinstate the connection.
You can also try it on all similar cables.
The keyway in the center of the socket means it can only go in one way.
This is the most common fault in electronics.
Second was ICs not making contact in their socket (same problem really).
If you press down firmly you might feel a little click as the IC reseats
For music stuff the next problem is usually dirt or wear in the encoder if it has been used a lot.
In old gear it is capacitors drying out. That is why everyone is re-capping their DX7 et al.
Royce